This invention relates to fuses and more specifically to methods for forming titanium tungsten fuses.
Programmable read only memories (PROMs) including titanium tungsten fuses are well known in the art. Typical prior art PROMs include an array of fuses. Each bit of data stored in the PROM is indicated by whether an associated fuse in the array is open or closed. Such PROMs are typically programmed by passing a current through selected fuses sufficient to melt portions of the titanium tungsten, thereby opening the fuse.
Titanium tungsten fuses are also used in programmable logic arrays (PLAs) to determine which of a number of logic functions is to be performed by the PLA. Such PLAs are described in "LSI Databook", 6th edition, published by Monolithic Memories, Inc., in 1985, incorporated herein by reference.
A typical prior art method for forming a titanium tungsten fuse is illustrated in FIGS. 1a through 1d. Referring to FIG. 1a, a silicon substrate 10 is covered with a silicon dioxide layer 12, a titanium tungsten layer 14, and an aluminum layer 16. Typically, semiconductor devices such as transistors, diodes and resistors (not shown) are formed in silicon substrate 10. Silicon dioxide layer 12 serves to insulate substrate 10 from titanium tungsten layer 14. Titanium tungsten layer 14 serves as a to-be-formed fuse and aluminum layer 16 serves as electrical interconnect material.
A photoresist layer 18 is then formed on aluminum layer 16. Photoresist layer 18 is then patterned, thereby exposing portions of aluminum layer 16. The exposed portions of aluminum layer 16 are then etched away, thereby exposing portions of titanium tungsten layer 14. Of importance, the etchant selected to etch aluminum layer 16 etches aluminum without etching the underlying portion of titanium tungsten layer 14. Typically, an etchant such as a mixture of acetic, phosphoric, and nitric acids is used during this step.
Thereafter, photoresist layer 18 is removed and the wafer is covered with a second photoresist layer 20 (FIG. 1b). Second photoresist layer 20 is patterned in a conventional manner, thereby exposing portions of titanium tungsten layer 14. The exposed portions of titanium tungsten layer 14 are then removed using a wet chemical etchant such as hydrogen peroxide (H.sub.2 O.sub.2). Thereafter, photoresist layer 20 is removed, thereby leaving the structure of FIG. 1c. FIG. 1d illustrates in plan view the structure of FIG. 1c.
As can be seen, the structure of FIGS. 1c and 1d includes metal 16 formed on titanium tungsten layer 14. A portion 14a of titanium tungsten layer 14 extends out from underneath aluminum layer 16. A narrow section 14b (typically about 1.5 microns wide) of portion 14a of titanium tungsten layer 14 serves as the fuse, which is opened when a current of about 15 to 30 mA is passed through it, depending on fuse dimensions.
In the plan view of FIG. 1d, aluminum layer 16 includes an aluminum interconnect 16a and aluminum pad 16b. Aluminum pad 16b typically electrically contacts the underlying semiconductor material via an opening in silicon dioxide layer 12 (not shown), while interconnect 16a electrically contacts other portions of integrated circuit.
Although the above-described prior art process provides adequate fuses, I have discovered that this process can be significantly improved as described below.